Field of the Invention
The present invention relates generally to methods and apparatus for processing interference due to non-linear products of transmitted signals in a wireless network, and more specifically, but not exclusively, to reduction of interference caused to a receiver due to passive intermodulation (PIM) products generated at more than one PIM source.
Description of the Related Technology
Non-linear products may be generated in a wireless network when one or more signals are transmitted along a signal path including a component having a non-linear transmission characteristic; these products differ in frequency from the signal or signals from which they were generated, and may potentially cause interference to other signals. The generation of non-linear products is becoming a problem of increasing importance in modern wireless communication systems, and in particular cellular wireless systems, since the radio frequency spectrum available has been steadily expanded as additional bands have become available, and the pattern of allocation of uplink and downlink bands within the available spectrum for use by various cellular systems, such systems using GERAN (GSM EDGE Radio Access Network), UTRAN (UMTS Terrestrial Radio Access Network) and E-UTRAN (Evolved UMTS Terrestrial Radio Access Network) radio access networks, and by various operators, is complex and territorially dependent. In this environment, and in particular at a cellular radio base station, it is likely that circumstances arise in which non-linear products generated from transmitted carriers in one or more downlink bands would fall within an uplink band in which signals are received at the base station. Non-linear products generated by non-linear characteristics of active components such as power amplifiers may generally be dealt with at a design stage by appropriate frequency management and filtering, but non-linear products caused by non-linear characteristics of passive components, for example passive intermodulation (PIM) products, may prove more difficult to manage. Many passive components may exhibit a non-linear transmission characteristic to some degree, for example due to an oxide layer at a metal to metal contact, and the non-linear characteristic may develop with time as an ageing process of the component. For example antenna and diplexer components, which are external to a receiver but connected to it, may generate PIM to some extent. Furthermore, PIM may be caused by components in the signal path between the transmitter and the receiver that are external to the transceiver equipment and which may be outside the operator's control, such as metallic objects on the antenna tower or other objects in the signal path through the propagation environment, such as fences; this is known as the “rusty bolt” effect.
Interference due to PIM may reduce the carrier to interference ratio at a receiver, which may reduce coverage by a cell significantly. The applicant's patent application GB2502279 addresses the reduction of interference caused by non-linear products of transmitted signals. Simulated interference signals are generated from a transmitted signal or signals, the simulated interference signals comprising simulated non-linear products of the transmitted signal or signals. The simulated interference signals may be correlated with a received waveform to detect whether or not the simulated non-linear products correspond to non-linear products that are present in received interference. The interference in the received waveform may be reduced by combining the simulated interference signals with received signals with an appropriate amplitude and phase relationship to cancel or reduce the interference.
In some circumstances the reduction of interference in the received waveform is limited when the interference comprises PIM from more than one source of PIM, in particular when the PIM sources are located at different distances from the receiver so that the round trip delay from the transmitter to the PIM source and back to the receiver is different for each PIM source.
It is an object of the invention to address at least some of the limitations of the prior art systems.